It is known in the art of electric drives for vehicles to provide electric drive systems which comprise a prime mover, a generator coupled to the prime mover and a traction motor connected to a driving wheel which receives power from the generator. Typical systems of this type generally fall into one of four categories: (a) the output voltage of the generator is rectified to direct current and then applied to a type of DC traction motor; (b) a hybrid system is utilized where the output voltage of the generator is rectified to obtain direct current for charging a battery, and the combination is then applied to a type of DC traction motor; (c) the output voltage of the generator is connected to a power-frequency AC energy source and then applied to a type of AC traction motor; and (d) in another hybrid system, the output voltage of the generator is rectified to obtain direct current for charging a battery, and a variable frequency inverter is used to convert the DC power into alternating current which is used to energize an AC traction motor that drives the vehicle.
One of the primary considerations in the designs of the above present-day automotive drive systems is the achievement of a fuel-efficient, low-contaminant drive source for automotive vehicles. The fuel efficiency of an internal combustion engine stays high only when operated at a constant speed and under constant load. It is known that when an ordinary motor car runs through a city area, the fuel efficiency drops to about one-third of the best value because it is necessary to frequently stop and start the car or to operate the engine under load and no-load conditions.
While systems of the types listed in the four categories above strive to meet the fuel efficiency demands that present restricted fuel supply conditions dictate, each system has certain drawbacks that frustrate the attainment of their efficiency goals.
A problem inherent in the use of DC motors such as those used in categories (a) and (b) are that these are bulky, heavy, and expensive in comparison to three-phase AC induction motors. An additional advantage of the AC motor is that no brush-commutator system is required.
The system of category (c), while not maintaining the prime mover and alternator at a constant RPM, rather controls the speed of the prime mover so as to be near-optimum for a manual throttle setting. A problem inherent in the design however, is that the prime mover RPM must be maintained at least 2.5 times the speed of the AC traction motor to prevent a malfunction in the power converter section of the system.
Inherent in the systems (b) and (d) employing a hybrid combination of a prime mover, generator, rectifier, and battery is that both energy sources (generator and battery) must be capable of supplying maximum energy to the traction motor. Even though in both cases, the prime mover and generator are operated at a constant RPM to minimize fuel consumption, the additional weight of the battery necessary to provide reasonable speed and driving range requires the provision of additional available energy from the two sources.
Solutions to the above listed problems are presented in the DISCLOSURE OF INVENTION section.
The prior art searched did not disclose any patents or publications that were directly related to an electromechanical propulsion system of the type disclosed herein. However, the following U.S. patents were considered in the investigation and evaluation of the prior art relative to the existing apparatus used with the invention:
______________________________________ U.S. PAT. NO. INVENTOR ISSUED ______________________________________ 3,551,685 Corry 12/29/70 3,792,327 Waldorf 2/12/74 4,113,045 Downing 9/12/78 4,119,861 Gocho 10/10/78 ______________________________________
The Corry Patent describes a motive power system for a vehicle comprising an engine-driven alternating current generator coupled to an AC electric drive motor through a power converter. The engine for driving the generator is an internal combustion engine and a control system is provided for operating the engine at near optimum fuel consumption over varying speeds of the engine.
The Waldorf Patent describes a vehicle electric drive including a DC traction motor powered by a prime mover-AC generator-rectifier-battery combination in which the traction motor is operated by the battery until the battery voltage drops to a level low enough to cause start-up of the prime mover-generator combination, thereby providing additional power for charging the battery and driving the traction motor.
The Downing Patent describes an all-electric AC tractor in which a prime mover operating at a constant speed is coupled to a three-phase alternator which, in turn, drives AC wheel motors through a cyclo-converter.
The Gocho Patent describes a starting aparatus for a gas turbine-generator mounted on an electric motor-driven vehicle. The electric motor vehicle is a hybrid type wherein the AC generator is DC-rectified and serves to charge a battery. The DC voltage is applied to an AC inverter which serves to drive AC wheel motors. The inverter drive frequency is controlled by a driver foot pedal.